How can I get my bisection method function to work?

I'm trying to write a bisection method algorithm to evaluate the minimum of a one variable function. I keep getting this error when i enter the command 'Fmin=bisection(4,6,10.^-3,F)':

"Undefined function 'bisection' for input arguments of type 'function_handle'. "

Can someone please help me figure out what I'm doing wrong?

My code is:

if true
  % code
end
function Fmin = bisection(a,b,e,F)
%BISECTION METHOD Input endpoints of starting and function to optimise over %four intervals and Fmin will output as local minimum.
xa=a;xb=b;xm=(a+b)./2;
while abs(xb-xa)>e
  F=feval(F,x);
  Fa=F(xa),Fb=F(xb),Fm=F(xm);
  xl=(xa+xm)./2,xr=(xm+xb)./2;
  Fl=F(xl),Fr=F(xr);
  y=[Fa,Fb,Fm,Fl,Fr];
  Fmin=min(y);
  if Fmin==Fa, 
      xb=xm,xm=xl,Fb=Fm,Fm=Fl;
  elseif Fmin==Fl, 
      xb=xm,xm=xl,Fb=Fm,Fm=Fl;
  elseif Fmin==Fb, 
      xa=xm,xm=xr,Fa=Fm,Fm=Fr;
  elseif Fmin==Fr, 
      xa=xm,xm=xr,Fa=Fm,Fm=Fr;
  elseif Fmin==Fm, 
      xa=xl,xb=xr,Fa=Fl,Fb=Fr;
  end
end
fprintf(Fmin)
end

I'm using this for function F:

if true
  % code
end
function F=f(x)
F=((x-1).^2).*((x-3).^2).*((x-5).^2)-5.*x;
end

Answer 1

[the cyclist]

I think you may have multiple problems with this code, but let's start at the beginning. Is your function saved in an M-file named bisection.m, in a directory in your path? What do you get if you type

>> which bisection

at the command line?

[Lulu]

When I first typed that command: 'bisection' could not be found.

I found where it was in the directory and added the folder to the path so when I entered it again I now get: C:\Users\Lulu\Documents\MATLAB\Numerical Optimisation\bisection.m

Running the code I still have an error but it is now:

Undefined function or variable 'x'.

Error in bisection (line 7)
    F=feval(F,x);

Thanks for helping!

[the cyclist]

If I understand your code correctly, you don't need that line that is giving you the error. Comment it out. The function F that you pass in will just operate directly on its arguments (like F(xa)).

I am calling bisection like this:

>> Fmin=bisection(4,6,10.^-3,@F)

Next, I get an error from your fprintf command. You could use

fprintf('%6.4f',Fmin)

or just use

disp(Fmin)

[Lulu]

Thank you, that has fixed it!